Bracken Cave is home to some 20million of the flying mammals – the largest colony on earth. That figure could double when the females are due to give birth in the prime breeding area throughout June. From April through to October, swarms of the Mexican Free-tailed bats nest in the protected area.

A VERY special visitor went and had a look:

He was NOT impressed. But he rarely is…

The area, near Austin, is something of a bat paradise – with the big-eared beasts gathering by their hundreds of thousands near a local bridge over the Lady Bird Lake.

Austin even has its own bat festival, Bat Fest, which takes place on Saturday 25 August this year.

But if you can’t wait that long to get your fix of the beasts, Bracken Cave is now open for public tours, which last for three to four hours, for the very first time.

‘It’s a spectacular sight. The bats form a vortex dense enough to show up on airport radar,’ Fran Hutchins, Bracken Bat Cave coordinator for Bat Conservation International, told MSNBC.

‘They fly in a 60-mile radius in search of food and in one night will eat hundreds of tons of insects, which makes the farmers around here very happy.’

The Mexican Free-tailed bat may not be the prettiest creature to look at but it’s one of the fastest.

According to Bats magazine, the expert long-range fliers have been recorded swooping through the air at 60 miles an hour, at altitudes of over 10,000 feet.

As well as Bracken Cave, they are found in Mexico and western North American and Central America.

And they don’t just live in caves – they will also nest and roost under bridges, in rock crevasses – even your house.

Not that you will want to encourage them as a family pet.

They are known for their huge quantities of guano, which contains billions of bacteria.

However, the droppings do have their uses.

Insecticides, detergents and antibiotics have all been made from the bats’ guano – as well is being used in gunpowder during the American Civil War.